上海交通大学：《操作系统 Operating System》课程教学资料（Java）Java Introduction.docx_大学文库

browsers soon incorporated the ability to run lava applets within web pages,and Java quickly became popular.With the advent of Java 2(released initially as J2SE 1.2 in December 1998 1999),new versions had multiple configurations built for different types of platforms.For example,2E targeted enterprise applications and the greatly stripped-down version J2ME for mobile applications (Mobile Java).J2SE designated the Standard Edition.In 2006,for marketing purposes,Sun renamed new12 versions as Java EE,Java ME,and Java SE,respectively. In 1997,Sun Microsystems approached the ISO/IEC ITCI standards body and later the Ecma International to formalize Java,but it soon withdrew from the process.Java remains a de facto standard,controlled through the Java Community Process.At one time,Sun made most of its Java implementations available without charge,despite their proprietary software status.Sun generated revenue from Java through the selling of licenses for specialized products such as the Java Enterprise System.Sun distinguishes between its Software Development Kit (SDK)and Runtime Environment (JRE)(a subset of the SDK);the primary distinction involves the JRE's lack of the compiler,utility programs,and header files. On November 13,2006,Sun released much of Java as free and open source software,(FOSS),under the terms of the GNU General Public License (GPL). On May 8,2007,Sun finished the process,making all of Java's core code available under free software/open-source distribution terms,aside from a small portion of code to which Sun did not hold the copyright. Sun's vice-president Rich Green said that Sun's ideal role with regards to Java was as an "evangelist."Following Oracle Corporation's acquisition of Sun Microsystems in 2009-2010,Oracle has described itself as the "steward of Java technology with a relentless commitment to fostering a community of participation and transparency".This did not hold Oracle,however,from filing a lawsuit against Google shortly after that for using Java inside the Android SDK(see Google section below). Java software runs on laptops to data centers,game consoles to scientific supercomputers.There are 930 million Java Runtime Environment downloads each year and 3 billion mobile phones run Java.On April 2,2010,James Gosling resigned from Oracle.2 Principles

browsers soon incorporated the ability to run Java applets within web pages, and Java quickly became popular. With the advent of Java 2 (released initially as J2SE 1.2 in December 1998 – 1999), new versions had multiple configurations built for different types of platforms. For example, J2EE targeted enterprise applications and the greatly stripped-down version J2ME for mobile applications (Mobile Java). J2SE designated the Standard Edition. In 2006, for marketing purposes, Sun renamed new J2 versions as Java EE, Java ME, and Java SE, respectively. In 1997, Sun Microsystems approached the ISO/IEC JTC1 standards body and later the Ecma International to formalize Java, but it soon withdrew from the process. [16] Java remains a de facto standard, controlled through the Java Community Process. [17] At one time, Sun made most of its Java implementations available without charge, despite their proprietary software status. Sun generated revenue from Java through the selling of licenses for specialized products such as the Java Enterprise System. Sun distinguishes between its Software Development Kit (SDK) and Runtime Environment (JRE) (a subset of the SDK); the primary distinction involves the JRE's lack of the compiler, utility programs, and header files. On November 13, 2006, Sun released much of Java as free and open source software, (FOSS), under the terms of the GNU General Public License (GPL). On May 8, 2007, Sun finished the process, making all of Java's core code available under free software/open-source distribution terms, aside from a small portion of code to which Sun did not hold the copyright. [18] Sun's vice-president Rich Green said that Sun's ideal role with regards to Java was as an "evangelist." [19] Following Oracle Corporation's acquisition of Sun Microsystems in 2009–2010, Oracle has described itself as the "steward of Java technology with a relentless commitment to fostering a community of participation and transparency". [20] This did not hold Oracle, however, from filing a lawsuit against Google shortly after that for using Java inside the Android SDK (see Google section below). Java software runs on laptops to data centers, game consoles to scientific supercomputers. There are 930 million Java Runtime Environment downloads each year and 3 billion mobile phones run Java. [21] On April 2, 2010, James Gosling resigned from Oracle. [22] Principles

One characteristic of Java is portability,which means that computer programs written in the Java language must run similarly on any hardware/operating-system platform.This is achieved by compiling the Java language code to an intermediate representation called Java bytecode, instead of directly to platform-specific machine code.Java bytecode instructions are analogous to machine code,but they are intended to be interpreted by a virtual machine (VM)written specifically for the host hardware.End-users commonly use a Java Runtime Environment (JRE) installed on their own machine for standalone Java applications,or in a Web browser for Java applets. Standardized libraries provide a generic way to access host-specific features such as graphics,threading,and networking. A major benefit of using bytecode is porting.However,the overhead of interpretation means that interpreted programs almost always run more slowly than programs compiled to native executables would.Just-in-Time (JIT)compilers were introduced from an early stage that compile bytecodes to machine code during runtime. Implementations See also:Free lava implementations Oracle Corporation is the current owner of the official implementation of the Java SE platform,following their acquisition of Sun Microsystems on January 27,2010.This implementation is based on the original implementation of Java by Sun.The Oracle implementation is available for Mac OS X,Windows and Solaris.Because Java lacks any formal standardization recognized by Ecma International,ISO/IEC,ANSI,or other third-party standards organization,the Oracle implementation is the de facto standard. The Oracle implementation is packaged into two different distributions: The Java Runtime Environment (JRE)which contains the parts of the Java SE platform required to run Java programs and is intended for end-users, and the Java Development Kit (JDK),which is intended for software developers and includes development tools such as the Java compiler, Javadoc,Jar,and a debugger. Open JDK is another notable Java SE implementation that is licensed under the GPL.The implementation started when Sun began releasing the Java source code under the GPL.As of Java SE 7,OpenJDK is the official Java reference implementation

One characteristic of Java is portability, which means that computer programs written in the Java language must run similarly on any hardware/operating-system platform. This is achieved by compiling the Java language code to an intermediate representation called Java bytecode, instead of directly to platform-specific machine code. Java bytecode instructions are analogous to machine code, but they are intended to be interpreted by a virtual machine (VM) written specifically for the host hardware. End-users commonly use a Java Runtime Environment (JRE) installed on their own machine for standalone Java applications, or in a Web browser for Java applets. Standardized libraries provide a generic way to access host-specific features such as graphics, threading, and networking. A major benefit of using bytecode is porting. However, the overhead of interpretation means that interpreted programs almost always run more slowly than programs compiled to native executables would. Just-in-Time (JIT) compilers were introduced from an early stage that compile bytecodes to machine code during runtime. Implementations See also: Free Java implementations Oracle Corporation is the current owner of the official implementation of the Java SE platform, following their acquisition of Sun Microsystems on January 27, 2010. This implementation is based on the original implementation of Java by Sun. The Oracle implementation is available for Mac OS X, Windows and Solaris. Because Java lacks any formal standardization recognized by Ecma International, ISO/IEC, ANSI, or other third-party standards organization, the Oracle implementation is the de facto standard. The Oracle implementation is packaged into two different distributions: The Java Runtime Environment (JRE) which contains the parts of the Java SE platform required to run Java programs and is intended for end-users, and the Java Development Kit (JDK), which is intended for software developers and includes development tools such as the Java compiler, Javadoc, Jar, and a debugger. OpenJDK is another notable Java SE implementation that is licensed under the GPL. The implementation started when Sun began releasing the Java source code under the GPL. As of Java SE 7, OpenJDK is the official Java reference implementation

The goal of Java is to make all implementations of Java compatible. Historically,Sun's trademark license for usage of the Java brand insists that all implementations be "compatible".This resulted in a legal dispute with Microsoft after Sun claimed that the Microsoft implementation did not support RMI or INI and had added platform-specific features of their own.Sun sued in 1997,and in 2001 won a settlement of US$20 million,as well as a court order enforcing the terms of the license from Sun.As a result,Microsoft no longer ships Windows with Java. Platform-independent Java is essential to Java EE,and an even more rigorous validation is required to certify an implementation.This environment enables portable server-side applications. Wikiversity has learning materials about Java Platform, Enterprise Edition/Java EE Tutorial Performance Main article:Java performance Programs written in Java have a reputation for being slower and requiring more memory than those written in C+t.However,Java programs execution speed improved significantly with the introduction of Just-in-time compilation in 1997/1998 for Java 1.1,the addition of language features supporting better code analysis (such as inner classes, the StringBuffer class,optional assertions,etc.)and optimizations in the Java virtual machine itself,such as HotSpot becoming the default for Sun's JVM in 2000.As of December 2012,microbenchmarks show Java 7 is approximately 44%slower than C++.1281 Some platforms offer direct hardware support for Java;there are microcontrollers that can run Java in hardware instead of a software Java virtual machine,and ARM based processors can have hardware support for executing Java bytecode through their Jazelle option. Automatic memory management Java uses an automatic garbage collector to manage memory in the object lifecycle.The programmer determines when objects are created,and the Java runtime is responsible for recovering the memory once objects are no longer in use.Once no references to an object remain,the unreachable memory becomes eligible to be freed automatically by the garbage collector Something similar to a memory leak may still occur if a programmer's code

The goal of Java is to make all implementations of Java compatible. Historically, Sun's trademark license for usage of the Java brand insists that all implementations be "compatible". This resulted in a legal dispute with Microsoft after Sun claimed that the Microsoft implementation did not support RMI or JNI and had added platform-specific features of their own. Sun sued in 1997, and in 2001 won a settlement of US$20 million, as well as a court order enforcing the terms of the license from Sun. [24] As a result, Microsoft no longer ships Windows with Java. Platform-independent Java is essential to Java EE, and an even more rigorous validation is required to certify an implementation. This environment enables portable server-side applications. Wikiversity has learning materials about Java Platform, Enterprise Edition/Java EE Tutorial Performance Main article: Java performance Programs written in Java have a reputation for being slower and requiring more memory than those written in C++. [25][26] However, Java programs' execution speed improved significantly with the introduction of Just-in-time compilation in 1997/1998 for Java 1.1, [27] the addition of language features supporting better code analysis (such as inner classes, the StringBuffer class, optional assertions, etc.), and optimizations in the Java virtual machine itself, such as HotSpot becoming the default for Sun's JVM in 2000. As of December 2012, microbenchmarks show Java 7 is approximately 44% slower than C++. [28] Some platforms offer direct hardware support for Java; there are microcontrollers that can run Java in hardware instead of a software Java virtual machine, and ARM based processors can have hardware support for executing Java bytecode through their Jazelle option. Automatic memory management Java uses an automatic garbage collector to manage memory in the object lifecycle. The programmer determines when objects are created, and the Java runtime is responsible for recovering the memory once objects are no longer in use. Once no references to an object remain, the unreachable memory becomes eligible to be freed automatically by the garbage collector. Something similar to a memory leak may still occur if a programmer's code

holds a reference to an object that is no longer needed,typically when objects that are no longer needed are stored in containers that are still in use.If methods for a nonexistent object are called,a "null pointer exception"is thrown. One of the ideas behind Java's automatic memory management model is that programmers can be spared the burden of having to perform manual memory management.In some languages,memory for the creation of objects is implicitly allocated on the stack,or explicitly allocated and deallocated from the heap.In the latter case the responsibility of managing memory resides with the programmer.If the program does not deallocate an object,a memory leak occurs.If the program attempts to access or deallocate memory that has already been deallocated,the result is undefined and difficult to predict,and the program is likely to become unstable and/or crash.This can be partially remedied by the use of smart pointers,but these add overhead and complexity.Note that garbage collection does not prevent "logical"memory leaks,i.e.those where the memory is still referenced but never used. Garbage collection may happen at any time.Ideally,it will occur when a program is idle.It is guaranteed to be triggered if there is insufficient free memory on the heap to allocate a new object;this can cause a program to stall momentarily.Explicit memory management is not possible in Java. Java does not support C/C++style pointer arithmetic,where object addresses and unsigned integers (usually long integers)can be used interchangeably.This allows the garbage collector to relocate referenced objects and ensures type safety and security. As in C++and some other object-oriented languages,variables of Java's primitive data types are not objects.Values of primitive types are either stored directly in fields (for objects)or on the stack (for methods) rather than on the heap,as commonly true for objects (but see Escape analysis).This was a conscious decision by Java's designers for performance reasons.Because of this,Java was not considered to be a pure object-oriented programming language.However,as of Java 5.0,autoboxing enables programmers to proceed as if primitive types were instances of their wrapper class. Java contains multiple types of garbage collectors.By default,HotSpot uses the Concurrent Mark Sweep collector,also known as the CMS Garbage Collector.However,there are also several other garbage collectors that can be used to manage the Heap.For 90%of applications in Java,the CMS Garbage Collector is good enough.au

holds a reference to an object that is no longer needed, typically when objects that are no longer needed are stored in containers that are still in use. If methods for a nonexistent object are called, a "null pointer exception" is thrown. [29][30] One of the ideas behind Java's automatic memory management model is that programmers can be spared the burden of having to perform manual memory management. In some languages, memory for the creation of objects is implicitly allocated on the stack, or explicitly allocated and deallocated from the heap. In the latter case the responsibility of managing memory resides with the programmer. If the program does not deallocate an object, a memory leak occurs. If the program attempts to access or deallocate memory that has already been deallocated, the result is undefined and difficult to predict, and the program is likely to become unstable and/or crash. This can be partially remedied by the use of smart pointers, but these add overhead and complexity. Note that garbage collection does not prevent "logical" memory leaks, i.e. those where the memory is still referenced but never used. Garbage collection may happen at any time. Ideally, it will occur when a program is idle. It is guaranteed to be triggered if there is insufficient free memory on the heap to allocate a new object; this can cause a program to stall momentarily. Explicit memory management is not possible in Java. Java does not support C/C++ style pointer arithmetic, where object addresses and unsigned integers (usually long integers) can be used interchangeably. This allows the garbage collector to relocate referenced objects and ensures type safety and security. As in C++ and some other object-oriented languages, variables of Java's primitive data types are not objects. Values of primitive types are either stored directly in fields (for objects) or on the stack (for methods) rather than on the heap, as commonly true for objects (but see Escape analysis). This was a conscious decision by Java's designers for performance reasons. Because of this, Java was not considered to be a pure object-oriented programming language. However, as of Java 5.0, autoboxing enables programmers to proceed as if primitive types were instances of their wrapper class. Java contains multiple types of garbage collectors. By default, HotSpot uses the Concurrent Mark Sweep collector, also known as the CMS Garbage Collector. However, there are also several other garbage collectors that can be used to manage the Heap. For 90% of applications in Java, the CMS Garbage Collector is good enough. [31]